Planar lightwave circuits (PLC) are well known in optical communications. They are formed on various substrates and include a network of waveguides, for example channel waveguides. An example of a PLC is shown in U.S. Pat. No. 6,507,680 issued Jan. 14, 2003 to Nishimura et al.
In the design of PLC architectures, it is important to properly integrate the taps and photodetectors with the optical transmission channels e.g. channel waveguides. In Nishimura, the photodetectors are integrated with the PLC and arranged for evanescent coupling.
It is also known to mount a photodetector in the path of a light beam for direct detection of a (tapped) light beam incident on the photodetector.
It is important to keep the photodetector(s) in a hermetically sealed housing as contaminants such as dust particles, water vapor or condensate, dust, fumes, smoke and other pollutants can adversely affect the photodetector's performance.
It is also desirable, when designing a planar lightwave circuit arrangement including a separate (i.e. not integral) photodetector (PD) or a PD array, to provide a spacing between the PLC and the PD array. The spacing should be sufficient to prevent a direct contact between the PLC and the photodetector array, but not excessive to avoid an undesirable divergence of a light beam tapped out of the PLC towards the PD array. The current embodiment utilizes a 200 μm spacing between the collimating plate and the photodetector array. This allows light to fill most of the photodetector element which is typically 80 μm in diameter. The light emerges from the waveguide at approximately 8 μm in diameter with a divergence angle of 12 degrees. The spacing between the collimating plate and photodetector allows the light to diverge to a 60 μm spot, thus filling most of the photodetector element. These dimensions can change depending on the photodetector element diameter selected. It is always desirable to fill at least a major part of the photodetector element, irrespective of the selected element nominal diameter. The secondary advantage of spacing the collimating plate from the photodetector array is that it prevents mechanical stresses from the outside surface of the collimating plate from being impeded into the photodetector array.
In an arrangement where the PD array is not an integral part of the PLC, the hermeticity requirement can be met by designing casings encompassing the entire PLC arrangement, i.e. the PLC with taps and the photodetector array. This however is a relatively costly solution. It is desirable to reduce the cost of a hermetic arrangement of the above-discussed type without sacrificing the hermeticity of the package and the quality of optical coupling between the PLC (specifically, the optical taps) and the respective photodetectors.
The prior art includes various examples of coupling between optical waveguides and photodetectors. U.S. Pat. No. 5,586,207 issued Dec. 17, 1996 to Northern Telecom describes methods and assemblies for packaging optoelectronic devices including a method of coupling an optical fiber to a packaged device using a collimating faceplate composed of parallel sections of optical fibers.
Collimating faceplates are also used in other arrangements, e.g. described in U.S. Pat. No. 6,160,606 to Sprague; U.S. Pat. No. 6,137,929 to Rosenberg et al; U.S. Pat. No. 6,318,909 to Giboney et al.; U.S. Pat. No. 5,170,455 to Goossen et al; and WO 02/39155 published May 16, 2002.